What I want to do is make it so that one of the playermodels can be Rorschach from watchmen. I already found a file of him for the source engine through a gmod mod. I just want to know how do I go about replacing this, so that I can see it in my game.

rename all the 'eightball' files to something like 'eightball_original' - make sure not to mess with those filename extensions (.sw.vtx, .phy etc). Then copy your model files in and rename them all to 'eightball'. Don't forget to copy the materials over as well, you won't need to rename them at all just make sure they are in the same directory as they were in gmod - create folders if you need to.

Then load up the game, select the character who is obviously not Nick from L4D2 and post screens back here!

I don't know, that decision was made before my time. I assume there was a good reason to do it though. You can find a bunch of source files on the public sync: http://sync.doubleactiongame.com/Models/Player if you would like to have a fiddle.